The scale on a map is 4cm : 6km. If the distance between two cities is 60km, how far apart in cm are the two cities on the map?
Explanation: The scale means that every 4cm on the map corresponds to an actual distance of 6km. An actual distance of 60km is the same as 10 $\cdot$ 6km. The distance between the two cities on the map is 10 $\cdot$ 4cm, or 40cm.